Dispersion is a known impairment in optical networks that causes a broadening of optical signals as they travel along the length of the fiber. One type of dispersion is chromatic dispersion (also referred to as “material dispersion” or “intramodal dispersion”), caused by a differential delay of various wavelengths of light in a waveguide material, i.e. the fiber. Disadvantageously, dispersion has a limiting effect on the ability to transmit high data rates, such as 10 Gb/s and 40 Gb/s. When modulated onto an optical carrier, an optical spectrum is broadened in linear proportion to the bit rate. The interaction of the broadened optical spectrum with wavelength-dependent group velocity (i.e., dispersion) in the fiber introduces signal distortions. The amount of tolerable distortion is inversely proportional to the bit rate. Thus, the combination of increasing spectral broadening and decreasing distortion tolerance makes the overall propagation penalty proportional to the square of bit rate. This results, for example, in a 10 Gb/s signal being 16 times less tolerant to dispersion than 2.5 Gb/s signal, while having only four times the bit rate. Dispersion accumulates linearly with propagation distance in the fiber and typical propagation distances in standard single-mode fiber (e.g., SMF-28 or equivalent) are about 1000 km at 2.5 Gb/s, 60 km at 10 Gb/s, and only 4 km at 40 Gb/s. Existing high-rate optical networks utilize some form of dispersion compensation to obtain meaningful propagation distances at bit rates of 10 Gb/s and above.
Methods to compensate for dispersion include fiber Bragg gratings, optical all-pass interference filters and dispersion compensating fiber. Dispersion compensating fiber (DCF) has found widespread practical acceptance and deployment due to its numerous advantages. Such advantages include relatively low loss and cost and the ability to simultaneously compensate channels across multiple wavelengths in wavelength division multiplexed (WDM) systems without requiring spatial separation. Further, DCF has the ability to compensate for the unavoidable variation in the dispersion as a function of wavelength (second-order dispersion or dispersion slope) that exists in many current transport fibers. Additionally at the individual signal level, dispersion can be pre-compensated utilizing frequency chirp induced by a modulator. Such pre-compensation can lead to reduced requirements for DCF and other external dispersion compensation methods.
High-speed fiber optic transmitters (e.g., 10 Gb/s, 40 Gb/s, etc.) utilize external modulators, such as a Lithium Niobate (LiNbO3) dual arm Z-cut modulators, to improve performance over direct modulators. Advantageously, external modulators allow sophisticated pre-distortion of an optical signal to compensate for dispersion and other non-linear effects experienced during transmission over a fiber. For example, external modulators can induce frequency chirp in the transmitted optical signal. Chirp is the broadening of the optical spectrum under intensity modulation. Chirp is a shift in light frequency (wavelength) as a result of a change in intensity level. Pre-Chirping uses fiber dispersion induced pulse broadening to narrow the pulse to overcome for dispersion experience over fiber transmission. Of note, pre-chirping can be positive or negative. Positively chirped pulses broaden more rapidly than zero chirp, and negatively chirped pulses initially narrow with the higher frequency light catching up. The sign of the chirp is selected according to the fiber plant to properly compensate for the natural broadening of a signal due to dispersion in a fiber by creating a compression effect. Various manufacturers provide fiber plant with different zero dispersion points for optimized transmission. Accordingly, the sign of the chirp is correlated to the zero dispersion point of the fiber plant.
The underlying optical signals include information in a digital format. Such information is typically formatted according to various standards, such as Synchronous Optical Network (SONET), Synchronous Digital Hierarchy (SDH), Optical Transport Network (OTN), and the like. Each standard includes overhead information accompanying data information, and one particular function of the overhead is to provide performance monitoring information. Additionally, such overhead can include Forward Error Correction (FEC) data used to improve optical performance by utilizing mathematical algorithms to proactively correct received bit-errors. For example, one such method of FEC is described in commonly-assigned U.S. Pat. No. 6,742,154 entitled “FORWARD ERROR CORRECTION CODES FOR DIGITAL OPTICAL NETWORK OPTIMIZATION.” In addition to optical signal improvement, FEC also provides a real-time metric of signal performance through a calculated bit-error rate (BER).